This invention relates to an array substrate, liquid crystal display device and their manufacturing method. More specifically, the invention relates to an array substrate having an inter-layer insulating film thick enough to suppress cross talk or coupling capacitance between wiring lines, preventing step-off breakage of electrodes at contact holes, and having a sufficient reliability; a liquid crystal device using the array substrate; and their manufacturing method.
The demand for high integration has progressively become strong regarding liquid crystal display devices such as an active matrix type display device. That is, it is required to make a number of various electronic elements and wiring lines in an extremely limited space.
To meet the requirement, multi-layer technologies are being brought into use. They are methods for stacking semiconductor layers and wiring layers via inter-layer insulation films. Multi-layer technologies enable to stack electronic devices and wiring lines without increasing the device area. Moreover, by making contact holes in the inter-layer insulating films to electrically connect upper and lower layers, complex wiring can be readily realized within a limited device area.
FIG. 12 is a schematic cross-sectional view of an array substrate 130 of a liquid crystal display device made by the multi-layer technique. The array substrate 130 shown here includes a poly-silicon (poly-crystalline silicon) layer 136 formed on a glass substrate 132 via an under-coat layer 134. A gate electrode 140 and an auxiliary capacity line 142 form a transistor and a capacitor. The source electrode 146 of the transistor is formed on an insulating film 144. The pixel electrode 154 is connected to the contact electrode 148 of the transistor via a contact hole 150a formed in the insulating film 150. A multi-layer technique is used in the layer under the source electrode 146 and the layer under the pixel electrode 154.
However, when a metal electrode and a semiconductor layer are stacked via an inter-layer insulating film as shown in FIG. 12, so-called cross talk occurs and causes an interference between signals which cannot be neglected. Moreover, an electrode layer and a semiconductor layer confronting with each other via an insulating film form a capacitor, and produces a coupling capacitance which causes problems, such as decrease of the response speed of the device.
For example, in the array substrate 130 shown in FIG. 12, the source electrode 146 supplies different signals as a signal line to other transistors not shown. However, as shown in FIG. 12 by the symbol A, the source electrode 146 and the pixel electrode 154 locally overlap via the inter-layer insulating film 150, and cross-talk or coupling capacitance occur in the overlapping portion. This causes problems, such as increase of noise components in a video signal, decrease of the video display quality due to a slow-down of the driving speed, and so on.
Additionally, wiring layers and other conductive layers on and under an inter-layer insulating film are patterned in many cases. Therefore, as the number of stacked layers increases, the surface unevenness becomes prominent, and makes it more difficult to form a multi-layered structure.
In order to prevent these problems, namely, an increase in cross-talk, coupling capacitance and surface unevenness, the inter-layer insulating film must be thick. However, conventional inter-layer insulating films made of inorganic materials are subject to cracking or peeling when they are thicker than hundreds of nm, and are unsuitable for use as thick films. On the other hand, inter-layer insulating films made of organic materials can be stacked thick, but cannot promise a sufficient reliability due to their high moisture permeability.
Another problem contained in multi-layered structures is step-off breakage of conductive layers at contact holes. Namely, when a conductive layer is stacked on an inter-layer insulating film having a contact hole, the thickness of the conductive layer becomes thinner on the inner wall surface of the contact hole, and readily broken there. For example, in FIG. 12, step-off breakage of the pixel electrode 154 is liable to occur at a position of the contact hole 150a shown by the arrow B. Step-off breakage inevitably causes insufficient connection between conductive layers resulting in low-quality video images of a liquid crystal display device. Step-off breakage more often occurs as the inter-layer insulating film 150 becomes thicker.
To prevent step-off breakage, the pixel electrode 154 must be thick. In a liquid crystal display deice, however, the pixel electrode 154 is typically a transparent electrode, and it is difficult to increase the thickness of the pixel electrode 154 due to a restriction by materials or its manufacturing process. Additionally, a thick conductive layer usually promotes the surface unevenness, and makes it difficult to form a multi-layered structure.